1. Field of the Invention
The invention relates generally to modified phenol-aldehyde resins and products incorporating such resins and, more particularly, the invention relates to the production of a modified phenol-formaldehyde resole resin useful in the manufacture of molded wood composites, e.g., fiberboard.
2. Brief Description of Related Technology
Man-made pressed boards, such as fiberboard, can be molded to have a three-dimensional shape and various design and structural features found in natural wood. Types of useful man-made boards are referred to by the following terms, for example: (a) fiberboards such as hardboard, softboard, and medium density fiberboard ("MDF") and (b) chipboards such as particleboard and oriented strandboard ("OSB"). Composites of these boards are also useful. These materials can be used to produce boards, siding materials, doorskins, and other structural or building products, for example.
Various processes can be used to produce wood composites such as those mentioned above. The principal processes for the manufacture of fiberboard, for example, include (a) wet felted/wet pressed or "wet" processes, (b) dry felted/drypressed or "dry" processes, and (c) wet felted/dry pressed or "wet-dry" processes. Synthetic resins, such as phenol-aldehyde resins, are often used as binders in these processes.
Generally, in a wet process, cellulosic fillers or fibers (e.g., woody material which is subjected to fiberization to form wood fibers) are blended in a vessel with large amounts of water to form a slurry. The slurry preferably has sufficient water content to suspend a majority of the wood fibers and preferably has a water content of at least 90 percent by weight ("weight percent"). The slurry is deposited along with a synthetic resin binder, such as a phenol-formaldehyde resin, onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of about fifty percent, based on the weight of dry cellulosic material. The wet mat is transferred to a press and consolidated under heat and pressure to form the molded wood composite.
A wet-dry forming process can also be used to produce wood composites. Preferably, a wet-dry process begins by blending cellulosic or wood fiber material in a vessel with large amounts of water. This slurry is then blended with the resin binder. The blend is then deposited onto a water-pervious support member, where a large percentage (e.g., 50 percent or more) of the water is removed, thereby leaving a wet mat of cellulosic material having a water content of about 40 wt. % to about 60 wt. %, for example. This wet mat is then transferred to a zone where much of the remaining water is removed by evaporation. The dried mat preferably has a moisture content of less than about 10 wt. %. The dried mat is then transferred to a press and consolidated under heat and pressure to form the wood composite which may be a flat board or a doorskin article, for example. The product can have any other desired shape depending on the intended use of the product.
In a dry process, the cellulosic fibers are generally conveyed in a gaseous stream or by mechanical means, rather than by a liquid stream. Cellulosic fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) can be first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, in a blowline blending procedure. The resin-coated fibers from the blowline can then be randomly formed into a mat by air blowing the fibers onto a support member. The fibers, either before or after formation of the mat, can optionally be subjected to pre-press drying, e.g., in a tube-type dryer. The mat, typically having a moisture content of less than 30 wt. % and preferably less than 10 wt. %, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
When medium density fiberboard production was introduced in the mid 1960s, fiber was first dried (e.g., in a drum type dryer) and then mixed with a resin such as a urea or urea-melamine resin, usually in blenders of the same type as are commonly used in particleboard plants. However, today, in the production of consolidated cellulosic articles, many processes include blowline blending of cellulosic material and a binder resin prior to the application of the dry process described above, for example. In blowline blending, the binder resin (generally saturated with steam from the fiberizing apparatus) is blended with the fiber with the aid of air turbulence. The blowline procedure takes place between the fiberizing apparatus and the pre-press dryer. Blowline blending offers several advantages, including ease, quality, and efficiency of blending of the fibers and the binder resin. For example, blowline blending (a) evenly distributes the binder throughout the fiber; (b) requires no special blending equipment; (c) reduces the amount of plant space and equipment necessary for equipment (e.g., for blenders); (d) obviates the down time necessary for blender cleaning; (e) allows for a smaller dryer tube diameter; (f) lowers the required temperature of the inlet to the dryer to which the blended material is sent; and (g) lowers the required temperature of the dryer to which the blended material is sent.
In many blowline processes, resins are applied to the wet fibers at about 200.degree. F.-250.degree. F. (about 93.degree. C.-121.degree. C.) (e.g., approximately 230.degree. F. or 110.degree. C.) and then are passed into a dryer (preferably a tube-type dryer) having an inlet temperature of about 320.degree. F. to about 400.degree. F. (about 160.degree. C. to about 200.degree. C). The inlet temperature depends upon dryer efficiency and/or the diameter of the dryer entry. Even though water evaporation holds the fiber temperature at an estimated 221.degree. F. (about 105.degree. C.) and the fiber is held at the elevated temperature in the dryer for only a matter of seconds, resins are put through quite a temperature shock. Depending on their reactivity and other properties, binders can lose a substantial amount of their efficiency or binding ability during this procedure.
When using any of the above-described processes, it is desirable that the binder resin be as efficient as possible. Resin efficiency, which can be affected by a number of factors, includes the ability to use a relatively low amount of resin for a given amount of filler. Resin efficiency is a particular concern when using blowline addition in a dry process.
One factor which will lower resin efficiency is the reactivity or precure of the resin, e.g., in the blowline or in the press prior to consolidation. To the extent that the resin precures (e.g., in the blowline due to the elevated temperatures therein), bonding efficiency can be lost and products manufactured with the resin may not perform satisfactorily.
Although many known phenolic resins have an advantage in that they are relatively slow curing (and therefore help to avoid undesirable precuring in the blowline), many of these phenolic resins can nevertheless exhibit a loss of efficiency while applied via a blowline procedure. Even phenolic resins which are very precure resistant (e.g., those which are produced at a low molar ratio of formaldehyde to phenol and with a low amount of catalyst) require more solids content when applied via blowline blending than when applied otherwise (e.g., via fiber blenders) for equal performance. It is therefore hypothesized that loss of resin efficiency is not solely caused by the resin's precure. Therefore, when selecting a resin for use in a blowline, not only is it desirable that the resin be resistant to precure, but it is desirable that the resin have other characteristics which make it suitable for blowline application.
For example, besides resin precure, loss of resin flow properties is another condition which can cause unsatisfactory performance of a resin in blowline application.
Loss of resin flow properties can be caused by "dryout," which is an insufficient amount of moisture in the system. Because resin flows much better in the presence of moisture, a very low moisture content can cause insufficient resin flow properties. Resin flow can be improved by increasing the amount of the moisture, however, this can be done only up to a certain level, since increased moisture content prevents resin cure.
In addition, it is hypothesized that advancing resin condensation during the drying step causes a loss of resin flow properties. Advancing resin condensation refers to the reaction between methylolated phenols (e.g., phenol reacted with formaldehyde) and/or between methylolated phenol and phenol itself (and/or between higher molecular weight species thereof). These reactions lead to higher molecular weight polymers. The higher the degree of condensation (the higher the molecular weight), the worse the resin flow.
In addition to dryout and advancing resin condensation, loss of resin flowability might also be caused by volatilization of the low molecular weight part of the resin. In particular, volatilization of free phenol (which acts as a resin plasticizer) under the hot and saturated conditions of the blowline can cause a loss of resin flowability.
Further, exposure of the resin to the hot expanded fiber in the dryer may cause excessive penetration of the resin into the wood fibers or particles. Such a phenomenon can also undesirably reduce the resin's efficiency.
When resin efficiency is compromised, if the resin level is not sufficiently raised to overcome the lower efficiency, products manufactured with the resin are likely to be of unsatisfactory quality. For example, molded wood composites made in such circumstances can have poor surface quality. Poor quality is indicated where the molded wood composite exhibits poor internal bonding and strength. Further, poor surface quality is shown where there appears to be layers within the composite which are "flaky" and can be easily peeled away. Where the wood composite has poor surface quality, the wood composite may easily break apart and the product is therefore unsatisfactory.
It is therefore desirable to produce a binder resin which performs satisfactorily under all pressing conditions, such as in a blowline in a dry process. More particularly, it would be desirable to produce a binder resin which has the following properties: (a) acceptable precure resistance against blowline blending conditions, (b) good flowability, (c) lack of excessive advancing resin condensation in the blowline, (d) lack of volatilization in the presence of steam, and (e) lack of overpenetration into the fiber (e.g., a resin having limited solubility). It is desirable to produce a wood composite product, e.g., fiberboard, which has significantly improved hardness of fiberboard surface, compared to commercial phenol-formaldehyde resins.
It is further desirable to produce a binder system which is convenient to use in a process such as those described above.